1. Field of the Invention
The present invention relates to a fuel injection control apparatus for internal combustion engines, which performs a learning control process to learn a difference between a fuel injection quantity and an instructed fuel injection quantity through a fuel injection valve.
2. Description of the Related Art
It is known for conventional diesel engines for vehicles to perform an additional fuel injection (such as a pilot injection, a pre injection, and a post injection) to inject a smaller quantity of fuel before or after performing a main fuel injection in order to suppress combustion noises and to decrease a quantity of NOx contained in an exhaust gas emitted from a diesel engine.
Because such an additional injection process injects a very small quantity of fuel per fuel injection, the fuel injection accuracy drastically decreases when a fuel injection quantity of an actual fuel injection is different from a fuel injection quantity of an instructed fuel injection (or a target fuel injection). As a result, it would be difficult to obtain desired effect generated by the additional fuel injection with a small fuel quantity which is performed before or after the main fuel injection.
In order to solve the above conventional problem, a conventional improved technique has been proposed, which performs a fuel injection quantity learning control. In the fuel injection quantity learning control, the conventional technique performs an additional fuel injection through a fuel injection valve, which is independent from the main fuel injection, during deceleration of a diesel engine when a fuel quantity of the main fuel injection becomes zero.
The conventional technique then estimates the fuel injection quantity in such an additional fuel injection based on the change of rotation speed of the diesel engine generated by performing the additional fuel injection, calculates a difference between the estimated fuel injection quantity and an instructed fuel injection quantity, and sets the difference value as a correction value (or a learning value) to the instructed fuel injection quantity. For example, Japanese patent laid open publication No. JP 2005-139951 has disclosed such a conventional technique.
However, the above conventional technique performs the fuel injection quantity learning control only during deceleration of the diesel engine (in other words, the timing to cut the fuel supply (“fuel cut timing”)) when the fuel injection quantity to be supplied to the diesel engine becomes zero. Therefore because the number of opportunities to learn the fuel injection quantity is small, it is difficult for the conventional technique to sufficiently perform the fuel injection quantity learning for the fuel injection valve of each of the cylinders of the diesel engine under a full injection condition before the difference between the fuel injection quantity and the instructed fuel injection quantity becomes within an allowable range
That is, the conventional fuel injection quantity learning control of the conventional technique described above calculates, as a learning value, the difference between the fuel injection quantity and the instructed fuel injection quantity generated in each of the cylinders per elapse of time. Because the difference between the fuel injection quantity and the instructed fuel injection quantity is changed according to the fuel pressure change to be supplied to the fuel injection valve, it is necessary to perform the fuel injection quantity learning control under such a full fuel injection condition.
However, the conventional technique has a fewer opportunities to perform the process of the fuel injection quantity learning control under the full fuel injection condition. Because it takes a lot of times to perform the fuel injection quantity learning control under the full injection condition by the conventional technique, it is difficult to update all of learning data (or correction data) by the elapsed time before a difference between a fuel injection quantity and an instructed fuel injection quantity of the fuel injection valve exceeds a value within the allowable range.